Why Does File.ReadAllLinesAsync() Block the UI Thread in .NET?

Asynchronous file I/O blocks the UI thread in .NET: Cause Analysis

The

File.ReadAllLinesAsync() method is designed to provide asynchronous file access, allowing the UI application to perform file operations in the background while maintaining smooth operation. However, as the code example shows, this method unexpectedly blocks the UI thread.

The reason for this behavior is inconsistent implementation of Microsoft's recommendations for asynchronous method design. File.ReadAllLinesAsync() Violates the principle of returning an incomplete Task after minimal synchronization work, and instead blocks the thread for a long time before returning.

Experimental results and optimization

Performance testing shows that File.ReadAllLinesAsync() may block the current thread for hundreds of milliseconds, seriously affecting UI responsiveness. This behavior is consistent across hardware configurations.

As a workaround, it is recommended to avoid using the asynchronous file system API in GUI applications. Instead, synchronous APIs should be wrapped in Task.Run to maintain asynchronous execution without blocking threads.

Improvements in .NET 6

While the asynchronous filesystem APIs have been improved in .NET 6, they are still not fully asynchronous and lag behind the synchronous APIs. Therefore, wrapping synchronous APIs in Task.Run remains the recommended approach for optimal UI performance.

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